Wireless sleep monitoring

ABSTRACT

The present invention provides a method and apparatus for physiological monitoring of a remote subject, the apparatus including: a base station having a transmission means for transmitting a reference signal; and a physiological monitoring probe connectable to the subject, the physiological monitoring probe having: receiver means for receiving the reference signal; monitoring means for monitoring the subject and generating a condition signal containing information related to a condition of the subject; modulation means for modulating the reference signal to produce a modulated reference signal containing the information contained in the condition signal; and passive retransmission means for passively retransmitting the modulated reference signal to the base station; wherein the base station has means for receiving the modulated reference signal, and means for demodulating the modulated reference signal to obtain the information related to a condition of the subject so that a condition of the subject can be monitored at the base station.

The present invention relates to apparatus and a method for remotemonitoring of the physiological state of a subject. The invention findsparticular application in the area of sleep apnoea and the inventionwill be described in relation to sleep apnoea, however, it is to beunderstood that the invention is not limited to the remote monitoring ofsleep apnoea.

Sleep apnoea occurs when a person stops breathing whilst asleep. Whilethe person is asleep, tissue in the air way relaxes causing the airwayto close and the person to stop breathing. Whilst the person is notbreathing the oxygen level in the person's blood falls until the brainwakes up and causes the air way to tighten allowing the person to breathagain. This cycle then repeats itself. People who suffer from sleepapnoea are likely to fall asleep during the day and may have otherassociated health problems.

If a person suffers from sleep apnoea or is suspected to suffer fromsleep apnoea it is often necessary for them to spend some time sleepingin a sleep laboratory so that their sleeping patterns and sleep apnoeacan be monitored. This is disruptive of the person's lifestyle.Furthermore, it is possible that the results obtained in a sleeplaboratory may not accurately reflect a person's sleeping patternsmerely because the person may not sleep normally in the unfamiliarsurroundings of the laboratory.

Accordingly, it would be advantageous to provide apparatus which allowsa person to be monitored in their own home. It would be preferable ifthe apparatus consumed little power so that its power source could besmall and/or recharged infrequently.

Accordingly, the invention provides apparatus for physiologicalmonitoring of a remote subject including:

-   -   a base station having a transmission means for transmitting a        reference signal; and    -   a physiological monitoring probe connectable to said subject,        said physiological monitoring probe having:        -   receiver means for receiving said reference signal;        -   monitoring means for monitoring said subject and generating            a condition signal containing information related to a            condition of said subject;        -   modulation means for modulating said reference signal to            produce a modulated reference signal containing said            information contained in said condition signal; and        -   passive retransmission means for passively retransmitting            said modulated reference signal to said base station;    -   wherein said base station has means for receiving said modulated        reference signal, and means for demodulating said modulated        reference signal to obtain said information related to a        condition of said subject so that a condition of said subject        can be monitored at said base station.

Preferably, the receiving means and passive retransmission means are apassive radio transponder.

Preferably, the monitoring means includes a physical parametertransducer.

Alternatively, the monitoring means may include a biological electrode.

The physiological monitoring means may include intermediate signal meansfor generating an intermediate signal derived by combining saidcondition signal with a fixed or varying frequency signal beforemodulating said reference signal.

Preferably fixed or varying frequency signal comprises a plurality ofsub-carrier signals.

The intermediate signal means may be operable to convert analog and/ordigital signals from the monitoring means to an intermediate signalwhich is used to modulate a radio frequency signal received by a passiveradio transponder, so that the transponder automatically retransmits amodulated signal which contains information relating to the condition ofthe subject.

The base station may include analogue and/or digital outputs foroutputting data.

Further, the base station may be connectable to a computer network, suchas the internet or world wide web, and operable to receive input(including configuration commands) and output data via said computernetwork. Thus, for example, any data displayable on a video display unitprovided in the base station could also be displayed on a remote videodisplay unit in communication with the base unit over a computernetwork. The outputs of the base station may therefore include worldwide web, WAP (Wireless Application Protocol) and/or satellitecommunications outputs.

The apparatus may optionally include encryption means so that saidapparatus can transmit data in encrypted form. Thus, data transmittedover such a computer network (or indeed by means of the reference signalbetween the base station and the physiological monitoring probe) may beencrypted so that the data is secure.

The modulated reference signal may comprise a synchronous or anasynchronous data signal.

Preferably said apparatus is operable to modulate the frequency or phaseof the reference signal by a Pseudo-Random Binary Sequence having aninstantaneous code that determines the respective instantaneousfrequency or phase.

The invention also provides a method of physiological monitoring of aremote subject including:

-   -   transmitting a reference signal from a base station to a remote        physiological monitoring probe connected to a subject;    -   monitoring said subject and generating a condition signal        containing information related to a condition of said subject;    -   modulating said reference signal to produce a modulated        reference signal containing said information contained in said        condition signal;    -   passively retransmitting said modulated reference signal from        said biological monitoring probe to said base station; and    -   demodulating said modulated reference signal to obtain said        information related to a condition of said subject so that a        condition of said subject can be monitored at said base station.

The method may include generating an intermediate signal derived bycombining said condition signal with a fixed or varying frequency signalbefore modulating said reference signal.

Preferably fixed or varying frequency signal comprises a plurality ofsub-carrier signals.

The method may include converting analog and/or digital signals from asubject monitoring means to an intermediate signal which is then used tomodulate a radio frequency signal received by a passive radiotransponder, whereby the transponder automatically retransmits amodulated signal containing information relating to the condition of thesubject.

The method may include transmitting data from said base station over acomputer network, such as the internet or world wide web, or inputtingdata (including configuration commands) over a computer network.

The method may include encrypting data to be output by said basestation, and/or encrypting said modulated reference signal so that thedata is secure.

The method may include transmitting said modulated reference signal as asynchronous or as an asynchronous data signal.

Preferably the method includes modulating the frequency or phase of thereference signal by a Pseudo-Random Binary Sequence having aninstantaneous code that determines the respective instantaneousfrequency or phase.

Preferably, the method is used to monitor sleep apnoea.

In order that the present invention may be more clearly ascertained, apreferred embodiment will now be described, by way of example, withreference to the accompanying figures in which:

FIG. 1 is a schematic diagram showing the relationship between a basestation and a plurality of physiological monitoring probes of aphysiological monitoring apparatus according to a preferred embodimentof the present invention;

FIG. 2 is a schematic diagram of a physiological monitoring probe of thea physiological monitoring apparatus of FIG. 1;

FIG. 3 shows a single channel modulation scheme for the physiologicalmonitoring apparatus of FIG. 1; and

FIG. 4 shows a multichannel modulation scheme for the physiologicalmonitoring apparatus of FIG. 1.

FIG. 1 shows a base station 1 and a plurality of physiologicalmonitoring probes 2, located at remote locations and connected in use toindividual subjects, of a physiological monitoring apparatus accordingto a preferred embodiment of the present invention. Hereinafter theoperation of the apparatus will be described in relation to a singlebase station and a single physical monitoring probe, however, it shouldbe understood that a single base station may be used to monitor aplurality of physiological monitoring probes and that the system couldemploy more than one base station.

The base station 1 is in radio communication with each physiologicalmonitoring probe 2, and so clearly includes an antenna (not shown).However, the base station 1 may include multiple antennas to improve thereception of the signal from each physiological monitoring probe 2.

Referring to FIG. 2, the physiological monitoring probe 2 includes apassive radio transducer 9, which itself includes an antenna 3 and apassive reflective modulator 4. When the base station 1 generates andtransmits a reference signal to the physiological monitoring probe 2,the signal is received by the antenna 3 and fed to the passivereflective modulator 4. The passive radio transducer 9 also acts as apassive radio transponder and thereby provides a passive retransmissionmeans. One or more physical parameter transducers or biologicalelectrodes 6 a, 6 b and 6 c provide signals to a signal conditioning andcontrol unit 5 which relate to a condition or physical indication of apatient or subject 10.

The signal conditioning and control unit 5 combines the signals receivedfrom the physical parameter transducers and/or biological electrodes 6a, 6 b, 6 c into a single data stream before feeding the data stream toa modulation means in the form of a passive reflective modulator 4 whereit is combined with a locally generated, fixed or varying frequencysub-carrier producing a complex intermediate signal before beingcombined with the radio frequency signal received from the base station1 to produce a modulated reference signal which is passivelyretransmitted to the base station unit 1. In an alternative preferredembodiment, however, the physiological monitoring probe 2 optionallygenerates two or more sub-carrier signals each containing signals fromthe physical parameter transducers and/or biological electrodes. Thisoption allows for a remote physiological monitoring probe 2 to use morethan one sub-carrier to transmit the modulated reference signals,thereby improving the capacity of the physiological monitoring probe 2to transfer information. It also allows the provision of additionalerror detection and correction by sending similar information via two ormore sub-carriers to reduce the chance of the information beingcorrupted by interfering signals. The duplicated or similar informationsent on each sub-carrier can be delayed in time for each sub-carrier toprevent short term bursts of interfering signals from corrupting theinformation.

Predominantly the energy used to generate the RF sidebands (containingthe monitored information) in the modulated reference signal can beextracted from the incoming radio energy of the reference signal.

The base station 1 subsequently demodulates the modulated referencesignal to extract information or data relating to the condition of thepatient therefrom, whereafter the extracted data can be used for one ormore purposes.

The physiological monitoring probe 2 also has a local power source 8such as a battery which can be rechargeable or an external power sourcesuch as photovoltaic panel. The power source 8 may have an electronicpower switch 7 driven by a toggling flip flop which can be toggled inresponse to a power switch command signal. In this way, when thephysiological monitoring probe is not in use it can be disabled to savebattery power. The command signal may be provided by the base station 1.Alternatively, the physiological monitoring probe could be equipped withan external accessible power switch.

The physiological monitoring probe 2 can be connected to the basestation 1 or to a storage cradle (not shown) temporarily so that it maybe re-programmed and/or configured or, where a rechargeable battery isprovided, so that the battery can be recharged. The physical monitoringprobe 2 may be configured via physical connections to the base stationor alternatively radio frequency, optical, inductive, capacitive orother means may be used to pass a signal from the base station 1 to thephysiological monitoring probe 2 to set various parameters for thatphysiological monitoring probe 2.

The apparatus my optionally include, for each physiological monitoringprobe 2, a storage holder (not shown). The physiological monitoringprobe 2 may be both recharged and remotely configured while in thisholder.

Parameters which may be set in this configuration procedure include:channel gains; channel bandwidths; channel number; identification forthe particular physiological monitoring probe; sub-carrier frequencies,and data channel rates. Information relating to the configuration of thephysiological monitoring probe 2 can be included in the modulatedreference signal transmitted from probe 2 so that the base station 1 canconfirm that the physiological monitoring probe 2 is correctlyconfigured.

The use of a passive reflective modulator 4 minimises power consumptionby eliminating the need for carrier frequency accuracy and stability andsignal generation in the passive radio transducer 9 thus extendingbattery life and reducing the required overall size of the physiologicalmonitoring probe 2.

The passive reflective modulator 4 has a resonant circuit which respondsto the incoming radio frequency signal (the reference signal) from thebase station 1, and contains a mixing device. Typically the mixingdevice consists of non-linear resistive or reactive components. Themixing device is fed with a locally produced intermediate signal fromthe signal conditioning and control unit 5 together with the referencesignal from the antenna. This signal is then impressed on the incomingreference signal which is a radio frequency signal, from the BaseStation, to create a modulated reference signal which is transmittedback to the base station.

The modulation method employed may be any appropriate analog or digitalmodulation system using for example: angle or amplitude methods ofmodulation.

The intermediate signal determines the offset or offsets of thesymmetrically modulated RF (radio frequency) side bands from the centrefrequency of the incoming RF reference signal by utilising differentsub-carrier frequencies for signals emanating from differentphysiological monitoring probes so that the probes can be distinguished.

FIG. 3 shows a modulation scheme for use when there is a single remoteunit (i.e. a single physiological monitoring probe 2). The base station(or unit) transmits a signal at a single carrier frequency. Thephysiological monitoring probe 2 combines the signal received from thebase unit with a locally generated intermediate signal containinginformation relating to the condition of a subject. This signal issubsequently passively retransmitted to the base unit where it can bedecoded to extract the information. In the multichannel situation shownin FIG. 4, the carrier frequency is received at both the first andsecond remote unit (i.e. at two physiological monitoring probes). Eachremote unit generates a different sub carrier signal containinginformation relating to a condition of the patient. The remote unitsthen separately retransmit their signals to the base unit which receivesthe signal containing both sub carrier signals.

In a multichannel system each remote unit (ie. each physiologicalmonitoring probe) which is within range of a base station is allocated aunique local sub carrier frequency. Hence, when more than one base unitis used the same sub carrier frequency can be assigned to more than onephysiological monitoring probe or each remote unit provided that remoteunit is only within range of one base station.

Where two or more independent base stations are required to operatewithin the range of remote units each base station's reference signalfrequency can be set to different values to the others by at least avalue of the typical resonant bandwidth of the remote units to preventremote units of one base station responding to the reference signal ofanother base station operating in the vicinity. This utilises frequencydivision multiplexing principles. The remote units therefore are tunedto operate on the particular frequency of the base station they areallocated, preventing them from responding to the other base station'sreference signal(s).

Similarly one base station could transmit more than one reference signalto operate with different sets of remote units while maintainingfunctional independence of the remote unit sets.

In a more complex multichannel variation the base station transmissionsignal frequency could be frequency (or phase) modulated by aPseudo-Random Binary Sequence (PRBS) whose instantaneous code determinesthe frequency (or phase) at that moment.

This moves the base station transmission signal frequency across asection of spectrum in a random manner, centred on the base stationtransmission centre frequency (fc) (this is a variant of what iscommonly called Spread Spectrum technology). By hopping or sliding thetransmission frequency in this manner, the base station received signalmoves synchronously with the transmission signal which greatlysimplifies the method of receiving and separating received signals.

To prevent signal corruption caused by interfering in-band signals, aPRBS method reduces the chance of the information sidebands occurring atthe same part of the spectrum as the interfering signals and thereforeinference is minimum. The individual information channels can beextracted with conventional methods once the received signal has thePRBS effects removed from the incoming signal. This method stillutilises a local intermediate signal to separate the individual channelsof the remote units associated with a signal base station—i.e. uniquelocal sub carrier frequencies are assigned to each physiologicalmonitoring probe.

Predominantly, the physiological monitor probe does not generate any RFenergy of its own but reflects the energy of the signal transmitted fromthe base station, back to the base station, modified with informationfrom the patient. The only energy used in the physiological monitoringprobe is that which is used to monitor the physiological condition andto combine the information signal with the intermediate sub carrier toproduce the intermediate signal. Predominantly the energy for the RFsidebands themselves can be extracted from the received signal so thatthe energy of the reference signal is dispersed between the reflected RFcarrier signal and the information side bands obtained from theintermediate signal without adding power to the RF signal from thephysiological monitoring probe.

The signal conditioning control unit 5 receives analogue and digitalsignals from one or more biological electrodes or physical parametertransducers 6 a, 6 b, 6 c which signals relate to physiologicalcondition of the subject or patient. The signal conditioning controlunit 5 converts these individual signals into a single digital oranalogue data stream which it feeds to its data modulator along with alocally generated local oscillator signal. The data modulator combinesthe local oscillator signal with the data stream producing an outputcomprising one or more sidebands which are centred on a sub-carrierderived from the locally produced oscillator signal. The information(data) signal which is modulated onto the sub-carrier is called theintermediate signal and is passed to the passive reflective modulator 4.The sub-carrier itself may have a spread spectrum to allow multipleremote units to modulate their respective information signals on the onesub-carrier frequency. The passive reflective modulator 4 combines theintermediate signal received from the signal conditioning and controlunit 5 with the incoming reference signal and passes the modulatedreference signal back to the antenna 3 of the passive radio transducer 9which re-radiates the signal across the radio path to the base station1. The intermediate signal from the signal conditioning control unit 5may also include other information such as: that required for forwarderror correction or other data detection or correction methods; framingcodes; data compression; delta modulation; an identification number;etc.

Preferably the antenna 3 is miniaturised. However, it may be eitherinternal or external to each physiological monitoring probe. In apreferred embodiment the antenna is integrated with the leads to thebiological electrodes or physical parameter transducers in order toreduce the required size of the unit. Depending on the application,different physical parameter transducers or biological electrodes may beused. That is to say, different transducers or electrodes may be useddepending on what condition of the patient is being monitored. Thetransducers or electrodes need not necessarily be formed integrally withthe physiological monitoring probe 2. For example, varying numbers ofelectrodes or transducers could be plugged in via leads to suitablegalvanic connections in the surface of the physiological monitoringprobe 2. In such an arrangement, separate power sources can provide foreach of the transducers or probes in order to reduce the burden on thepower source 8 which is used to drive the passive radio transducer 9.

The base station 1 has one or more radio receivers, a power supply whichmay be either mains power or a battery, one or more data demodulators,one or more data processing modules, a variety of external interfacessuch as indicator lights and displays and data inputting means such askeyboard or buttons. Optionally, the base station 1 may be connectableto a computer network, such as the internet, so that its outputs can bemonitored at locations other than that of the base station 1, and sothat data can be entered into the base station 1 from those locations.

The radio receiver has means for pre-processing the received signalbefore it is fed to the data demodulator. The radio receiver alsoincludes one or more radio frequency amplifiers, one or more localoscillators, one or more mixers, one or more filters, one or moreattenuators, one or more matching networks and one or more intermediatefrequency amplifiers.

The data demodulator uses a demodulation method which complements themethod used to modulate the reference signal in the physiologicalmonitoring probe 2.

The data processing module receives data from the demodulator andinterprets this data so that individual data channels can be separatedwhich relate to different conditions of the person. Error detection canbe performed on the data at this stage and the data can be sent to theexternal interfaces or stored in a data storage device or both.

The base station also includes a control module which is responsible forthe overall operation of the base station. The control module configuresand/or controls and/or operates a radio receiver for frequency, gain, RFpower, control of the data demodulator configuration and/or operation.Indicators and displays are also controlled by the control module. Forexample the control module may monitor the data from the data moduleuntil it detects a particular piece of data or pattern of data whichindicates the need to sound an alarm. For example, if the device isbeing used to monitor sleep apnoea an alarm may be sounded if the personstops breathing for an extended period of time. Connection of the basestation 1 or a physiological monitoring probe 2 via an interface toexternal devices allows, in one embodiment, the detected condition toactivate intervention equipment to reduce or prevent the condition fromoccurring for dangerous periods.

Other external interfaces may be utilised which display and/or recordmonitored physiological data for diagnosis.

The base station 1 can also include means for indicating faults such asloss of received signal from the physiological monitoring probe 2.

The physiological monitoring apparatus hereinbefore described can beused for a number of applications such as the monitoring of thecondition of a patient and alternatively as a vigilance monitor foroperators of vehicles or equipment. While the physiological monitoringapparatus has been described in relation to monitoring sleep apnoea, itwould be used to monitor any number of physiological conditions of asubject where it is appropriate that the subject be monitored remotely.Physical conditions which could be monitored include, but are notlimited to:

-   -   pulse rate, blood pressure (in real time or as an average over        time), temperature (external and internal), humidity (internal        and external), skin conductivity, EEG, EMG, ECG, flexure,        movement, skin colour changes, pulse oximeter, plethysmography,        airflow (nasal and oral) and fluid flow.

Depending on what monitoring is being undertaken, a different set ofconditions can be monitored. For example, vigilance monitoring ofdrivers will require the monitoring of a different range of physicalconditions to sleep apnoea. Furthermore, in some circumstances it may behelpful to monitor some aspects of the environment in which the subjectis located, for example: temperature, levels of gases such as oxygen,the amount of light to which the subject is exposed, etc. These aspectscan be monitored by the physiological monitoring probe attached to thesubject, although in some circumstances it may be appropriate to useseparate monitoring equipment.

The system can incorporate further features and has additionaladvantages as discussed below.

The simplicity of the system allows it to be small in size have lowweight, low cost and consume less power. The consumption of less powerallows batteries to be of a small size and for them to last for theduration of a monitoring session. If a patient is under constantmonitoring batteries can be changed or recharged at longer intervals.The small size also allows the physiological monitoring probe to be wornas a small pad or patch with the sensors in the form of biologicalelectrodes or physical parameter transducers 6 a, 6 b, or 6 c beingattached or integral to the physiological monitoring probe. Because thesystem is wireless it does not encumber the patient. Similarly, becausethe system can be light in weight, it will not encumber the patient.Another feature of the system can be that the transmission power can bebelow licensing limits therefore obviating the need for radio frequencylicensing of the system. As discussed above, the multichannel nature ofsome embodiments of the system allows there to be several remote unitsfrom a signal RF channel and allows more remote units of physiologicalmonitoring probes to be added to the system at any time. Anotheradvantage of the system is that it allows the patient to move aroundeasily because no cables have to be disconnected. With an appropriatedesign and choice of RF frequency the electrode wires of thephysiological monitoring probe 2 can be combined with the antenna 4 inorder to reduce the size of the device. Similarly the antenna may bemade integral to the physiological monitoring probe itself. There isalso potential for the device to be modified so that it can be implantedin a patient's body.

Again, as discussed above, the present system is not limited tomonitoring of sleep apnoea and can be used for various physiological andphysical monitoring processes. Also, as previously discussed the systemis not limited to use on patients and as such can be used in othercircumstances where it is necessary to monitor a subject. The systemcould also be used to monitor animals.

Modifications within the spirit and scope of the invention may readilybe effected by a persons skilled in the art, so it is to be understoodthat this invention is not limited to the particular embodimentsdescribed by way of example hereinabove.

1. Apparatus for physiological monitoring of a remote subject including:a base station having a transmission means for transmitting a referencesignal; and at least one physiological monitoring probe connectable tosaid subject, said physiological monitoring probe or probes having:receiver means for receiving said reference signal; monitoring means formonitoring said subject and generating a condition signal containinginformation related to a condition or conditions of said subject;intermediate signal means for generating an intermediate signal derivedby combining said condition signal with a fixed or varying frequencysub-carrier signal before modulating said reference signal; modulationmeans for modulating said reference signal with said intermediate signalto produce a modulated reference signal containing said informationcontained in said condition signal; and passive retransmission means forpassively retransmitting said modulated reference signal to said basestation; wherein said base station has means for receiving saidmodulated reference signal, and means for demodulating said modulatedreference signal to obtain said information related to one or moreconditions of said subject so that at least one condition of saidsubject is available to be monitored at said base station, and said basestation includes means for varying the frequency or phase of thereference signal so that said reference signal is a spread spectrumreference signal.
 2. Apparatus as claimed in claim 1, wherein saidreceiving means and passive retransmission means are a passive radiotransponder.
 3. Apparatus as claimed in claim 1, wherein said monitoringmeans includes a physical parameter transducer.
 4. Apparatus as claimedin claim 1, wherein said monitoring means includes a biologicalelectrode.
 5. Apparatus as claimed in claim 1, wherein said intermediatesignal means is operable to convert analog and/or digital signals fromthe monitoring means to an intermediate signal which is used to modulatea radio frequency signal received by a passive radio transponder, sothat the transponder automatically retransmits a modulated signal whichcontains information relating to the condition of the subject. 6.Apparatus as claimed in claim 1, wherein said passive radio transponderuses a plurality of intermediate signals to modulate a radio frequencyreference signal.
 7. Apparatus as claimed in claim 1, wherein said basestation includes analog and/or digital outputs for outputting data. 8.Apparatus as claimed in claim 1, wherein said base station isconnectable to a computer network, and operable to receive input andoutput data via said computer network.
 9. Apparatus as claimed in claim1, including encryption means so that said apparatus can transmit and/orreceive data in encrypted form.
 10. Apparatus as claimed in claim 1,wherein said condition signal includes a synchronous or an asynchronousdata signal.
 11. Apparatus as claimed in claim 1, wherein said basestation is operable to vary the frequency or phase of the referencesignal by a continuously varying signal having an instantaneous valuethat determines the respective instantaneous frequency or phase. 12.Apparatus as claimed in claim 11 in which the continuously varyingsignal is derived from a Pseudo-Random Binary Sequence.
 13. A method ofphysiological monitoring of a remote subject including: transmitting areference signal from a base station to at least one remotephysiological monitoring probe connected to a subject; varying thefrequency or phase of said reference signal so that said referencesignal is a spread spectrum reference signal; monitoring said subjectand generating a condition signal containing information related to acondition or conditions of a said subject; generating an intermediatesignal derived by combining said condition signal with a fixed orvarying frequency sub-carrier signal; modulating said reference signalwith said intermediate signal to produce a modulated reference signalcontaining said information contained in said condition signal;passively retransmitting said modulated reference signal from saidbiological monitoring probe to said base station; and demodulating saidmodulated reference signal to obtain said information related to thecondition or conditions of said subject so that the condition orconditions of said subject can be monitored at said base station.
 14. Amethod as claimed in claim 13, wherein said intermediate signal is oneof a plurality of intermediate signals, and said fixed or varyingfrequency sub-carrier signal is one of a plurality of sub-carriersignals, each corresponding to a respective one of said plurality ofintermediate signals.
 15. A method as claimed in claim 13 furtherincluding converting analog and/or digital signals from a subjectmonitoring means to the intermediate signal which is then used tomodulate a radio frequency signal received by a passive radiotransponder, whereby the transponder automatically retransmits amodulated signal containing information relating to the condition of thesubject.
 16. A method as claimed in claim 13, including transmittingdata from said base station over a computer network, and/or inputtingdata over a computer network.
 17. A method as claimed in claim 13,including encrypting data to be output by said base station, and/orencrypting said modulated reference signal.
 18. A method as claimed inclaim 13, including transmitting said condition signal as a synchronousor an asynchronous data signal.
 19. A method as claimed in claim 13,including varying the frequency or phase of the reference signal by acontinuously varying signal having an instantaneous value thatdetermines the respective instantaneous frequency or phase.
 20. A methodas claimed in claim 19 in which the continuously varying signal isderived from a Pseudo-Random Binary Sequence.
 21. A method as claimed inclaim 13, wherein said method is used to monitor sleep apnoea. 22.Apparatus as claimed in claim 1, wherein said base stations is alsooperable to use a fixed frequency reference signal.